An inertial navigation system (INS) is a widely used technology for guidance and navigation of a vehicle. The INS is composed of an inertial measurement unit (IMU) and a processor where an IMU houses accelerometers and gyroscopes which are inertial sensors detecting platform motion with respect to an inertial coordinate system. An important advantage of the INS is independence from external support, i.e., it is self-contained. However, the INS cannot provide high accuracy at long ranges. Particularly, inertial sensors are subject to errors that tend to accumulate over time, i.e., the longer the drive time, the greater the inaccuracy.
More recent developments in global positioning system (GPS) have made high accuracy vehicle navigation possible at low cost. However, since the GPS relies on GPS satellites, it is susceptible to jamming, RF (radio frequency) interference and multipath problems. Although the GPS provides accurate position and velocity over longer time periods, the GPS involves occasional large multipath errors and signal dropouts. Therefore, efforts are made to develop an integrated INS/GPS navigation system by combining the outputs of a GPS and an INS using a Kalman filter to remedy performance problems of both systems.
Inertial sensors used to be expensive and bulky only used in precision application, for example, aerospace and military navigation. For establishing an IMU package in a compact and inexpensive manner, efforts have been made to develop microelectro mechanical systems (MEMS) sensors. When a ground vehicle is, for example, in a downtown area of a city where there are many tall buildings, the navigation system receives GPS signals not only through direct paths but also multiple paths because of reflections of the GPS signals by the buildings as shown in FIG. 1. Such multipath GPS signals present problems (hereafter “mutipath problems”) in a navigation system because the time of arrival of the GPS signal varies depending on the length of the path taken until reaching the GPS receiver.
Numerous researchers are studying mitigation of GPS multipath problems for a GPS receiver all by itself. While many of them try to solve this problem by modifying the correlation process internally in a GPS receiver's algorithm, some propose handy logics using a receiver's output signals to eliminate measurements including multipaths. To the best of the inventor's knowledge, there is no economical and practical solution for multipath problems applicable to integrated INS/GPS systems where the worst case performance should fall in the INS-only performance when GPS performance degradation occurs.
Therefore, there is a need of a new method to solve the GPS multipath problems in an economical and practical way for a MEMS based integrated INS/GPS navigation system which is utilizing low-cost, small, but noisier MEMS inertial sensors.